1. Field of the Invention
The present invention relates to an optical technique that can be applied, for example, to a display such as liquid crystal display.
2. Description of the Related Art
Liquid crystal displays have characteristics of thin-shaped, lightweight and low power consumption. Thus, in recent years, their application to mobile devices and stationary equipments such as television receivers increases rapidly.
In order to make it possible for a liquid crystal display to display a multi-colored image, a color filter is utilized. For example, in a transmissive or reflective liquid crystal display that can display a multi-colored image, a color filter including red, green and blue coloring layers is utilized in most cases. On the other hand, in a semi-transparent liquid crystal display that can display a multi-colored image, a color filter including red, green and blue coloring layers for transmissive display and red, green and blue coloring layers for reflective display is utilized in most cases.
Many liquid crystal displays include a retardation layers. For example, in a liquid crystal display of a television receiver, a retardation layer is utilized in combination with a linearly polarizing film in order to display an image that can be easily recognized regardless of the viewing direction. On the other hand, in a reflective or semi-transparent liquid crystal display, an absorption-type circularly polarizing plate including a quarter-wave plate or a combination of a quarter-wave plate and a half-wave plate as a retardation layer is utilized in order to achieve an excellent visibility under a high-luminance light source such as sun.
However, in spite of the fact that the red, green and blue pixels are different in wavelength range of display color from one another, the retardation of a retardation layer is usually even throughout its surface. For this reason, it is difficult to adopt optimal designs into all the pixels different in display color.
In addition, each of the retardation of a liquid crystal layer and the retardation of a retardation layer has wavelength dispersion. For this reason, when employing a design for sufficiently compensating the retardation of a liquid crystal cell using a retardation layer at pixels that display a certain color, the retardation layer may insufficiently compensate the retardation of a liquid crystal cell at pixels that display other colors.
Furthermore, in the case where a quarter-wave plate, which causes a retardation by a quarter of a wavelength (λ/4) at a center wavelength of green wavelength range, for example, about 550 nm, is combined with a linearly polarizing plate to be used as a circularly polarizing plate, even when the refractive index anisotropy, i.e., birefringence Δn of the quarter-wave plate is almost uniform throughout the wavelength range of visible rays, a retardation greater than λ/4 will be caused within a blue wavelength range having a center wavelength of, for example, about 450 nm. Also, a retardation smaller than λ/4 will be caused within a red wavelength range having a center wavelength of, for example, about 630 nm. Thus, when the circularly polarizing plate is irradiated with blue and red lights as natural lights, the transmitted light will be not a circularly polarized light but an elliptically polarized light. In fact, since the birefringence is greater on the short-wavelength's side of the visible range, i.e., within the blue wavelength range and is smaller on the long-wavelength's side of the visible range, i.e., within the red wavelength range, this problem is often more serious.
In view of the above-described problems, JP-A 2005-24919 and JP-A 2006-85130 propose as a retardation layer a solidified liquid crystal layer that includes regions having different thickness, i.e., regions causing different retardations.
Specifically, JP-A 2005-24919 describes that a color filter layer composed of red, green and blue coloring layers different in thickness is formed, and a solidified liquid crystal layer is formed on the color filter layer. The solidified liquid crystal layer is obtained by coating an alignment layer with a coating solution containing photo-polymerizing liquid crystal compound and irradiating the coated film with ultraviolet rays.
According to this method, due to the relief structure that the coloring layers produces on the surface of the color filter layer, a solidified liquid crystal layer thicker at a position of the thinner coloring layer and thinner at a position of the thicker coloring layer can be obtained. That is, a solidified liquid crystal layer different in thickness among pixels that displays different colors can be obtained. In other words, a solidified liquid crystal layer including regions that cause different retardations can be obtained.
JP-A 2006-85130 describes a semi-transparent liquid crystal display that includes a color filter layer and a solidified liquid crystal layer. In this liquid crystal display, each coloring layer of the color filter layer is thicker at the transmissive portions of pixels and thinner at the reflective portion of the pixels. That is, the surface of the color filter layer is provided with a relief structure. The solidified liquid crystal layer is obtained by forming a polyimide layer on the surface of the color filter layer provided with the relief structure, performing a rubbing process on the whole surface of the polyimide layer, coating the polyimide layer with ultraviolet-curing liquid crystal monomer, and irradiating the coated layer with ultraviolet rays. Alternatively, coating the surface of the color filter layer with a liquid crystal polymer and subjecting the whole of the coated film to a photo-alignment process obtain the solidified liquid crystal layer. The solidified liquid crystal layer thus obtained is thinner at the transmissive portions of pixels and thicker at the reflective portions of the pixels. That is, according to the method, a solidified liquid crystal layer that includes regions causing different retardations can be obtained.
However, according to the technique described in JP-A 2005-24919, it is necessary to accurately adjust the differences in thickness among the coloring layers. Similarly, according to the technique described in JP-A 2006-85130, it is necessary to accurately adjust the difference between the thickness of the coloring layer at the reflective portion and the thickness of the coloring layer at the transmissive portion. For this reason, when the above-described techniques are employed, the design for the color filter layer is limited or the degree of difficulty in manufacturing the color filter layer increases. Therefore, in order to achieve the design thickness at each region of the solidified liquid crystal layer, it is necessary to consider various factors such as flowability of a coating solution and a shrinkage ratio of the coated film.
JP-A 2008-505369 (KOHYO) proposes a biaxial oriented film having periodically varying local birefringence. The film described therein is a short pitch cholesteric film, and develops additional in-plane anisotropy (Δnx-y) in the negative C-type structure, due to the helical strain. More specifically, a drawing indicates the development of an index ellipsoid which satisfies nx≠ny≠nz, wherein nx and ny are greater than nz, and has biaxial negative C-type symmetry.
According to the description, the film is produced by, for example, irradiating the material with linearly polarized light, preferably linearly polarized UV light to induce the photoreaction of a photosensitive compound in a selected region of the material. In the film, the helical structure is uniform, but the birefringence varies locally throughout the helix.